1. Technical Filed of the Invention
The present invention relates to a light beam scanning apparatus generating a scanning light beam and an image forming apparatus, and in particular, to a light beam scanning apparatus in which sequential digital image data read from an object to be copied is distributed into a plurality of strings of data which are modulated based on image data string by string and the modulated results are synthesized to be used for control of the generation of laser light, and an image forming apparatus with this laser light scanning apparatus therein.
2. Related Art
Recently, a various types of imaging forming apparatuses, such as digital copying machines and laser printers, have been developed and are now already in practical use, in which exposure scanning that employs a laser light beam (hereinafter, simply called “light beams”) is synthesized with an electrophotography process to form images.
This image forming apparatus is based on the principle that light beams are simultaneously scanned and exposed on and along a single photosensitive drum to form a single electrostatic latent image on the photosensitive drum and the electrostatic latent image is copied onto a sheet of paper.
In the field of the image forming apparatuses, especially in recent years, there has been a strong demand for forming images at higher speeds. In order to respond to this demand, a technique described in Japanese Patent Application No. 2004-168425 has been proposed. In this configuration, a single laser oscillator generating a scanning light beam is disposed and a plurality of transfer channels to the laser oscillator are formed to transfer image data thereto. Practically, a data processor is provided, in which image data supplied from a scanner section are subjected to predetermined image processing and digital image data of each line are distributed and outputted into two strings of image data (i.e., two systems (channels) of data): a string of image data at odd-number-th pixels (odd pixels) and a string of image data at even-number-th pixels (even pixels). This data processor further includes two serial circuits each having a PWM (pulse width modulator) and a laser driver, which process the image data composing each string. Both the laser drivers, each belonging to each data transfer channel, have output terminals electrically connected to the single laser oscillator via, for example, a wired logical add (OR) circuit.
However, in the case of the foregoing configuration, various drawbacks which must be removed have been pointed out. These drawbacks, which concern with the modulation function of the PWM, will now be detailed in connection with FIGS. 1 to 3.
The PWM has a function of making a pulse width and a pulse position for each pixel changeable. Changing the pulse width allows an image to be formed smaller than one pixel or a line to be formed finer. Further, changing the pulse position produces smooth oblique lines with less irregularity.
FIG. 1 shows how to change both the pulse width and the pulse position, which are processing to be performed together with the pulse with modulation at the PWM. Bits of information to change both the pulse width and the pulse position are given to the PWM as image data from a processor located on the upstream side of the PWM. The pulse position, which is not free to select, is selected from a left reference (front reference), right reference (rear reference), and central reference of one pixel.
Employing the left reference as the pulse position is to place the reference position at the front edge of each pixel. Hence, as a pulse width is made larger, the pulse extends toward the right side (i.e., toward the rear side) (refer to t1 to tMax in FIG. 1(A)). In contrast, employing the right reference as the pulse position is to place the reference position at the rear edge of each pixel. The pulse thus extends toward the left side (i.e., toward the front side), as the pulse width is made larger (that is, the rear edge of the pulse agrees to the rear end of each pixel: refer to t1 to tMax in FIG. 1(C)). When the central reference is employed as the pulse position, the pulse extends toward both sides thereof, with the center of the pulse made to agree to the center of each pixel (refer to t1 to t5 in FIG. 1(B)).
Assume that a modulation circuit is used, in which two PWMs (PWM1 and PWM2) are arranged in parallel and their outputs are synthesized with each other. In this modulation circuit, for producing images at faster speeds, the odd pixels are produced by the PWM1, while the even pixels are produced by the PWM2. The PWM1 for the odd pixels uses the left reference and/or the central reference in performing the pulse width modulation, but the PWM2 for the even pixels uses the central reference and/or the right reference in performing the pulse width modulation. In the pulse width modulation, each of the PWM1 and PWM2 operates on the reference clock T1, as shown in FIG. 1. However, the PWM1 and PWM2 are assigned to the production of the odd and even pixels, respectively, and their outputs are synthesized with each other.
How to synthesize the modulated outputs is shown in FIG. 2. In synchronism with a horizontal synthesizing signal BD, the modulated outputs (image data) of the PWM1 and the modulated outputs (image data) of the PWM2 are outputted as image data PIXDAT1 and PIXDAT2, respectively. Both the image data PIXDAT1 and PIXDAT2 are synthesized with each other to produce image data PIXDAT3. This image data IPIXDAT3 becomes a drive signal repeated at a high speed based on a cycle of T2 (=T1/2). This drive signal is outputted to the laser driver as a laser drive signal, with the result that the laser driver switches on/off the laser for scanning in a main scanning direction (the axial direction of the photosensitive direction) at the cycle of T2 in which the synthesized image data PIXDAT3 is reflected. Accordingly, the speed of the image formation is doubled.
However, in above configuration in which the plural PWMs are arranged in parallel, the following drawback has been pointed out.
The odd pixels are formed with the use of the left reference and/or the central reference assigned to the PWM1. That is, the position the right edge (rear edge) of each odd pixel is decided using the central reference to the PWM1, but the central reference will cause the pulse to extend toward both the sides thereof, with the central position of the pulse kept as a reference. Hence, as understood, there is caused a drawback that the pulse is obliged to run off the center and extend toward the neighboring even pixel. Meanwhile, the even pixels are formed by using the central reference and/or right reference assigned to the PWM2. Since the position of the left edge (front edge) of each even pixel is decided using the central reference to the PWM2, there is a problem that the pulse is obliged to run over the center and extend toward the neighboring off pixel.
FIG. 3 exemplifies pulses (image data PIXDAT2) based on the central reference to the PWM2 for producing the even pixels, in which the pulses are ran over toward the respective neighboring odd pixels. In this case, pulses (image data PIXDAT3-A) produced by synthesizing pulses (image data PIXDAT1) based on the left reference to the PWM1 and pulses (image data PIXDAT2) based on the central reference to the PWM2 loses the boundary between each odd pixel and each even pixel.
In this example, as illustrated by “PIXDAT-B,” a desired synthesized pulse is found in a pulse train in which a pulse shown by encircled 1 and a pulse shown by encircled 2 are separated from each other. This separation will lead to printing, for example, a vertical line of ¾ pixel width, a blank of ¼ pixel width, and a vertical line of ¼ pixel width on sheets of paper.
However, the running over of the pulses (image data PIXDAT2) toward the odd pixels will cause a pulse train in which each pulse shown by the encircled 1 and each pulse shown by the encircled 2 are connected, with no separation therebetween. Hence repeating the scanning based on the pulse train shown by “PIXDAT-A” over a plurality of lines eliminates the blank of ¼ pixel width, which should originally be formed between the vertical line of ¾ pixel width and the vertical line of ¼ pixel width, whereby a vertical line of 5/4 pixel width is printed.